In a medical device that monitors and/or treats ventricular arrhythmia, the invention provides for a "normal" processing mode of operation and a "guarded" processing mode of operation. The transition between the two modes is a function of cardiac rhythm. Benign rhythms are processed in the normal processing mode and non-benign rhythms are processed in the guarded processing mode. In normal processing mode, the device performs functions that are not computationally demanding, but in guarded processing mode, the device performs the computationally demanding operations that classify arrhythmias. If therapy is deemed necessary, the device may provide the therapy in guarded processing mode.